Hydrogen firing at elevated temperatures to change the color of glasses whose compositions contain reducible ions is well known. A notable commercial application of that technique is found in the Coming Incorporated eyewear product lines marketed under the SERENGETI.RTM. and CPF.RTM. trademarks. The color changes induced are attributed, to the reduction of a portion of the silver and lead ions respectively, in the glass to the atomic state.
Several methods have been suggested for making polarizing glasses. For example, one such method is to redraw a glass above its softening temperature. The glass contains a separate phase which is elongated by the redraw process. The thermal treatment which leads to the phase separation is usually carried out before the redraw process. In a particular version of the above process, the separated phase is initially spectrally non-absorbing material such as AgClBr, CuClBr, AgI, CuI or copper/cadmium halides, which must be subsequently modified to create a desired dichroic property necessary for the polarizing effect. This is accomplished by treating the stretched glass in hydrogen gas at elevated temperatures for sufficient time to effect the chemical reduction of the spectrally non-absorbing materials to their corresponding metal. The chemical reduction process is a combined process involving both the diffusion of hydrogen in the glass, and the chemical reaction of the hydrogen with the halide phase.
It is known that the chemical reaction proceeds very fast relative to the hydrogen diffusion which leads to the condition of a sharp boundary between the reduced region near the surface, and the unreduced region below the surface. The polarizing behaviors derives from the reduced layer. Also, when the polarizing glass is heated to the vicinity of 500.degree. C. for any prolonged period of time, the elongated particles re-spheridize and the polarizing property is lost. That is, the elongated particle returns to its spherical shape. This is explained by the fact that once the glass is soft enough, the interfacial forces act to undo what the redrawing forces had accomplished.
For certain applications, it is desirable to have the polarizing properties restricted to localized regions of the glass. Accordingly, it is the object of the present invention to provide methods of partially or fully blocking the effect of hydrogen reduction over a portion of a glass surface, or other ways of altering the polarizing state.